The present invention relates to a scanning optical apparatus which is used, for example, as a laser scanning unit of a laser beam printer, and more particularly to a technique for preventing occurrence of displacement of a focusing point due to temperature change.
The scanning optical apparatus generally employs a laser diode and a collimating lens that collimates a laser beam emitted by the laser diode. When ambient temperature of members securing the laser diode and the collimating lens changes, the securing members may expands and/or contracts and positional relationship therebetween and/or with respect to the entire optical system may change, and a focusing point (a point on which the beam is focused by the optical system) may deviated in a direction of the optical axis.
Japanese Patent Provisional Publication No. SHO 59–15206 discloses a structure which avoids such a problem. According to the teaching, the laser diode and the collimating lens are secured using a double-layer structure using two members with different thermal expansion coefficients, and displacement of the collimating lens due to thermal expansion of one member and displacement of the collimating lens due to thermal expansion of the other member are cancelled out by each other.
The scanning optical apparatus also employs a scanning lens (e.g., fθ lens). When a glass lens is used as the scanning lens, it is sufficient that a distance between a light emitting point and the collimating lens is controlled to be kept constant.
Recently, a plastic lens is often used as a scanning lens (e.g., fθ lens) in order to reduce a manufacturing cost. If the plastic lens is used as the scanning lens, thermal expansion of the scanning lens and a change in refractive index of the scanning lens due to the temperature change cannot be ignored. That is, when the plastic scanning lens is employed in the scanning optical system, the structure disclosed in the above publication may not compensate for the deviation of the focusing point sufficiently.
When the temperature increases, the focal length of the plastic scanning lens increases, and thus, the focusing point is shifted in a direction away from the scanning lens. On the other hand, when the temperature increases, the distance between the light emitting point and the collimating lens increases if they are secured on a single securing member. As the distance between the light emitting point and the collimating lens increases, the beam emitted by the laser diode cannot be collimated by the collimating lens, rather a converging beam is emerged from the collimating lens. In such a case, the focusing point is shifted in a direction toward the scanning lens. Furthermore, when the temperature increases, the wavelength of the laser beam emitted by the laser diode is shifted in a longer side.
Japanese Patent No. 2736984 discloses technology in which, in consideration of change in characteristics of the plastic scanning lens and wavelength variations of the laser beam due to the temperature change, deviation of the focusing point due to the temperature change is suppressed by utilizing thermal expansion of a securing member for securing both the laser diode and the collimating lens. By using aluminum or zinc as a material for the support member, the deviation of the focusing point due to temperature change is compensated for.
It should be noted that since the optical system of the scanning optical apparatus constitutes a magnifying optical system which is a combination of a collimating lens with a short focal length and a scanning lens with a long focal length, a change in distance between a light emitting point and the collimating lens due to temperature change is magnified on an image side, resulting in the deviation of the focusing point. Therefore, the position of the collimating lens to the securing member has to be managed at high accuracy. For this purpose, according to configuration described in the above patent publication, the double-layer structure is employed, in which the collimating lens is attached to a lens frame and the lens frame is attached to the securing member.
In the conventional scanning optical apparatuses described above, however, the double-layer structure having a relatively complex securing member is employed. Further, the securing member is formed of a metal such as aluminum or zinc. Therefore, the structure of an optical scanning unit also becomes complex and expensive.